Although several billion pounds of pesticide are used annually in the United States to control various pests, these pests are still responsible for annual farm production losses equivalent to 30 percent of the total market value of farm products. Part of the problem is due to the lack of full availability of the pesticide to the target area. Environmental conditions such as wind drifting, as well as degradation and evaporation of applied pesticide lead to low efficiency.
Some of the lost efficiency is due to the inherent conflicts between pesticide application and the action mode of the pesticide. For example, many contact pesticides are water soluble and require a certain minimum moisture level before they will move from the soil surface to the subsurface where they can be effective. Other agriculturally effective materials, like water soluble herbicides and fertilizers, must move from the soil or plant foliage surface to the subsurface before they can be effective. The solubility of the material limits the use of area spraying in favor of broadcasting dry solids.
When applied as solids to the soil surface, however, there may be some time delay before sufficient moisture is available to transport the applied materials. This surface exposure is potentially harmful as extended periods of ultraviolet light can significantly degrade many organic chemicals and, since many such chemicals are toxic, nontarget organisms (e.g., birds, rodents, etc.) can inadvertently teed on the applied solids.
Various methods have been used to achieve more efficient pesticide usage. Such methods include protecting the active agent by encapsulated formulations, including controlled release formulations thereof. U.S. Pat. No. 3,565,818 describes a multistep process for encapsulating a core material from a liquid solution containing polyvinyl alcohol and an alkylene glycol cyclic borate ester. As described, the process involves forming the PVA-borate ester liquid phase, adding core material particles to the liquid, and treating the capsules with an aqueous solution of a transition metal salt such as vanadyl sulfate.
U.S. Pat. No. 3,574.133, like the '818 patent, describes a multistep process for producing microcapsules through liquid-liquid phase separation using an aqueous solution of a polyvinyl alcohol and a cyclic borate ester of an alkylene glycol. As described, the process involves forming the PVA-borate ester liquid phase, adding core material particles to the liquid, dehydrating the carrier to encapsulate the core, and optionally treating the capsules with a crosslinking transition metal salt.
U.S. Pat. No. 3,582.495 describes treating polyvinyl alcohol-walled capsules with aqueous vanadium ions. The so-treated capsules are substantially water-insoluble and exhibits a substantial degree or impermeability for materials contained within the capsule walls. In the process disclosed in U.S. Pat. No. 3,629,140, the water solubility of polyvinyl alcohol-walled capsules previously hardened by hydrous vanadyl is re-established by treatment with a material which oxidized the vanadium. The encapsulated material can then be released in water.
U.S. Pat. No. 4,439,488 describes a method of encapsulating chemical biological agents by entrapment within a polyhydroxy polymer borate matrix. In the disclosed process the chemical biological agent is dissolved or dispersed in an aqueous paste of a polyhydroxy polymer. The agent to be encapsulated can be added either prior to or subsequent to pasting of the polyhydroxy polymer. Suitable gel-forming polyhydroxy polymers include natural, derivatized and modified starches, carboxymethylcellulose, dextran, xylan and polyvinyl alcohol. The addition of boric acid or a boric acid derivative, at an alkaline pH, converts the paste into a gel, thereby entrapping the agent in a protective matrix. The gel is then sheared into small particles, alehydrated, sieved and dried to yield a friable granular or powdered material. Rewetting of the product is said to initiate biodegradation of the matrix and release of the active ingredient.
U.S. Pat. No. 4,440,746 describes a slow-release soil pesticide composition. The composition is prepared by dispersing the pesticide in an aqueous solution of polyvinyl alcohol (PVA). An aqueous solution of a borate compound is then added to the PVA-pesticide dispersion and reacted with stirring until a gel is formed. The gel is then treed from water by drying. The dried product is then ground and past through a mesh screen to obtain particles of a desired size. The product is to be applied to soil where it release the pesticide.
U.S. Pat. No. 4,777,089 and U.S. Pat. No. 4,908,233 describe a polyvinyl alcohol-based microcapsule made by adding a substantially water-insoluble core material to an aqueous solution of a polyvinyl alcohol polymer which undergoes phase separation upon the addition of an electrolyte. Addition of the electrolyte causes phase separation whereby the aqueous polymer solution encloses the core material. Suitable electrolytes include salts of inorganic acids such as sulfuric acid, hydrochloric acid, phosphoric acid, boric acid and vanadic acid, and salts of organic acids. Sulfates and phosphates are disclosed as being the preferred salts. Ionic surfactants and amino acids may also be used as the electrolyte. After completion of the phase separation, the capsule wall films are gelled by cooling and then dried to be used as capsules soluble in water. Exemplified core materials include cleaning agents, perfumes and foods. The core component is released by dissolution of the microcapsules when diluted with water during use. In an alternative embodiment, the wall films may be hardened to prepare water-insoluble capsules. Disclosed hardening agents include borax and water-soluble salts of vanadium and uranium. After drying, the capsules are broken to release the core material.
U.S. Pat. No. 5,160,530 describes a process for encapsulating herbicidal compositions (e.g., the yellow polymorphic form of trifluralin with a melt point of 43.degree.-45.degree. C.) by melting or grinding the active ingredient (AI), combining the AI with an aqueous solution containing one of a number of possible film forming polymers, and spray drying the mixture at a temperature within the range from about 500.degree.-220.degree. C.
Japanese patent 74/048073 describes a method for dispersing a hydrophobic substance, such as ink, pigment, perfume, into a solution containing polyvinyl alcohol and then hardening the shell with a hardening agent such as borax.
One might think that a nonaqueous carrier liquid could be used to spray the solids over the treatment area. Unfortunately, nonaqueous dispersion carriers are generally more expensive than water and certainly less readily available. Nonaqueous liquids have also been the subject of a heightened degree of regulatory scrutiny in recent years with the general regulatory preference away from such carriers.
It would be desirable to have an encapsulation formulation for water soluble active ingredients that could be used with an aqueous carrier liquid yet release the encapsulated active ingredient at a target location when sufficient moisture is present to transport the encapsulated active ingredient.
It would also be desirable to have an encapsulation formulation that would permit the addition of additives, including ultraviolet screening agents, to protect UV sensitive active ingredients until moisture is available for dissolution and transport away from the exposed application surface.